Generally, dialkylbenzenes obtained by the dealkylation of benzenes are a mixture of 1,2-, 1,3- and 1,4-isomers, but a difference in boiling point between these isomers is so small that, in many cases, even rectifying columns having many plates are insufficient to separate these isomers from one another by distillation. Next, one specific example will be given. Cymene isomers obtained by the alkylation of toluene with propylene have the following boiling points: o-Isomer, 178.3.degree. C.; m-isomer, 175.1.degree. C.; and p-isomer, 177.1.degree. C. A difference in boiling point between m- and p-cymenes, which comes into special question in the cresol manufacturing process, is only 2.degree. C., so that separation of the both by rectification is extremely difficult. In the cresol manufacturing process now in use, therefore, the following procedure is employed: The mixed cymene, without being separated into the isomers, is oxidized as such into a mixed cresol (in this oxidation, the oxidation rate of o-cymene is very slow as compared with m- and p-cymenes so that the mixed cresol obtained consists mainly of m-cresol and p-cresol in general), and thereafter, separation of the cresol isomers is carried out.
As one method to separate the cresol isomers from one another, there is a one in which the cresol mixture is alkylated with isobutylene into a mixture of tert-butyl cresol isomers, the isomers are separated from one another by rectification taking advantage of a large difference in boiling point between them, and then the tertiary butyl group is eliminated to obtain high-purity m- and p-cresols.
As another method to separate the cresol isomers from one another, there is a one in which a mixture of cresol/urea isomeric clathrate compounds is separated into the isomers by recrystallization taking advantage of a difference in crystallizability between them, and the separated compounds are decomposed to obtain high-purity m- and p-cresols.
The foregoing both methods are a separation method now in use in industry, but their process is so complicated that a furthermore improvement is desired.
Another specific example will be given below. Diisopropylbenzene obtained by the alkylation of benzene, which is a starting material for 1,3-dihydroxybenzene (resorcinol) and 1,4-dihydroxybenzene (hydroquinone), comprises the isomers having the following boiling points: o-Isomer, 200.degree. C.; m-isomer, 203.2.degree. C.; and p-isomer, 210.3.degree. C. A difference in boiling point between m- and p-diisopropylbenzenes, which comes into special question in the resorcinol and hydroquinone manufacturing process, is 7.degree. C., so that separation of the both by rectification is possible. This method, however, requires rectifying columns having a fairly large number of plates so that it may not always be said to be a separation method of good efficiency.
Instead of these conventional separation methods, there are proposed ones based on a new idea which are intended to selectively dealkylate only the 1,4-dialkyl isomer in the dialkylbenzene, to thereby recover the 1,3-dialkyl isomer (in some cases, 1,2- plus 1,3-dialkyl isomers) as unreacted (Japanese Patent Application (OPI) Nos. 93716/1980 and 83721/1980). (The term "OPI" as used herein refers to a "published unexamined Japanese patent application", hereinafter the same). This method uses a ZSM type zeolite as a catalyst, and particularly, a ZSM type zeolite catalyst modified with oxides such as MgO, P.sub.2 O.sub.5, etc. will dealkylate only the 1,4-dialkyl isomer with a very high selectivity, so that this method is a markedly epoch-making technique.
From the practical point of view, however, this method also has one large defect that, when the alkyl group to be dealkylated has not less than three carbon atoms, olefins obtained by the dealkylation are low in purity and percent recovery. For example, Example 10 of Japanese Patent Application (OPI) No. 83716/1980 discloses that m-cymene is obtained in a high purity (96.6%) by dealkylating a mixed cymene (o:m:P=2.16:66.16:31.67) using a steam-treated H-ZSM-5, but the content of recovered propylene in the volatile gas obtained at that time is about 60%. Similarly, Example 11 of Japanese Patent Application (OPI) No. 83721/1980 discloses that a high-purity m-cymene is obtained by dealkylating a mixed cymene using a similar catalyst, but the content of propylene recovered at that time is 43%. In the above two cases, the volatile gas contains C.sub.2 -C.sub.5 hydrocarbons in addition to propylene. When an industrial process is taken into account, olefins produced by dealkylation (propylene in the case of dealkylation of cymene) need to be re-used by recycling to the alkylation zone, otherwise the material cost becomes too high to establish a practical industrial process. When the olefin purity is however low, the olefin is difficult to re-use as it is, and therefore separate olefin-purification equipments become necessary, which is a defect of this method.
On the other hand, Japanese Patent Application (OPI) No. 103119/1981 discloses that when the reaction is carried out in the presence of an H-ZSM-5 catalyst while feeding a mixed cymene and aniline or ammonia, the dealkylation proceeds with a high para-selectivity, whereby propylene is recovered in a high purity as 94%. Though this method is an excellent technique, it has the following defects: Namely, when an actual embodiment to be applied industrially is taken into account, while the recovered propylene is recycled into the alkylation region, if a base such as aniline or ammonia is entrained, the dealkylating catalyst becomes deactivated. Thus, it is necessary to separate aniline or ammonia and to purify propylene, which results in rendering the process not economical.